Parker O-Ring Handbook.pdf

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Dynamic O-Ring Sealing 5-6 are designed to ride on the hydraulic system during transit. Transient pressures of 690 Bar (10,000 psi) are not uncommon in these cases. A mechanical lock or brake should be provided to hold a position once it is attained. The hydraulic cylinder should be used only to raise and lower the load if it does not have a relief valve within it to prevent excessive pressure build-up by shock loads. 5.8 High Frequency Motion or Vibration O-rings or other seals can be worn excessively by small frequent motions which are usually encountered when equipment is in transit. For example: the tilt cylinder of a lift truck, a hydraulic tailgate lift, and a road scraper blade. Normally, the hydraulic cylinder is intended as an actuator and not as a locking device or a snubber. It will be noted that brick pavements and dirt roads cause the most trouble when this type of effect is encountered. A mechanical lock is also recommended as a cure in this case. 5.9 Squeeze The best squeeze for a reciprocating O-ring seal must be a compromise of all the factors involved. The design tables in this chapter are generally satisfactory. The greater the temperature range to be sealed, the greater the squeeze that is needed. The same is true if low pressure or vacuums are encountered. On the other hand, too much squeeze will cause excessive friction, wear, and occasionally spiral failure. Some rubber compounds require more squeeze than others in order to seal. The nitrile (buna-N) base compounds are recommended whenever possible because they are more extrusion-resistant, more wear-resistant, and require less squeeze to seal, than any other oil-resistant rubber developed to date. The military services have found that more than 0.432 mm (0.017") squeeze (per side) on a 5.334 mm (0.210") cross section makes an O-ring prone to spiral failure. Yet much less than this amount of squeeze will allow leakage at low temperature. As discussed before, the amount of squeeze is a vital factor in friction. Therefore, one should carefully consider the squeeze applied to the O-ring in any gland design. Squeeze is actually necessary only during periods of very low or no pressure sealing because at high pressures the O-ring seeks the path of least resistance, the clearance gap, and tends to seal tighter and tighter as the pressure is increased. Enough squeeze must always be provided to offset the great difference in coeffi cient of shrinkage of the rubber and the metal, take up the tolerances of the metal and rubber parts, and compensate for the shrinkage (if any) of the rubber in the fl uid. The following example illustrates how the squeeze can vary in a typical piston installation: Parker O-Ring Handbook Consider Parker size 2-012 and Design Table 5-2: 1. With perfect concentricity Gland Depth, Lmax = 0.501 - 0.387 = 0.057 2 Radial clearance, max = 0.501 - 0.496 = 0.0025 2 Cross section, Wmin = .067 Reduction of W, due to installation stretch = 0.003 (see Figure 3-3) Wmin, installed = 0.064 less Lmax = 0.057 (from 1. above) squeeze, min = 0.007 2. With maximum radial displacement (piston tangent with bore) squeeze, min = 0.007 (from 1. above) radial piston shift, max = 0.0025 squeeze = 0.0045 min possible 3. With maximum eccentricity of 0.002 T.I.R. between piston and groove OD squeeze, min = 0.0045 (from 2. above) radial piston shift, max = 0.0010 squeeze, min. = 0.0035 with adverse tolerance build-up. If the O-ring is made in a compound that will shrink in the fl uid, the minimum possible squeeze under adverse conditions then must be at least .076 mm (.003"). 5.10 Stretch When an O-ring must be stretched more than two or three percent as installed in a piston groove, the reduction in the squeeze diameter that results should be allowed for in determining the gland depth so that the desired percent squeeze will be applied to the reduced section. The percent of stretch should therefore be checked whenever the catalog gland dimensions are not used. Large diameter O-rings may fi t the piston so loosely that they must be carefully stuffed into the groove as the piston enters the cylinder to prevent damage. For these, the danger of damage is reduced if the next smaller size O-ring is used. Since this will likely cause a stretch close to fi ve percent, it will usually be necessary to adjust the gland depth as mentioned above. See Figure 3-3 for the reduction in squeeze diameter with stretch. Parker Hannifi n Corporation • O-Ring Division 2360 Palumbo Drive, Lexington, KY 40509 Phone: (859) 269-2351 Fax: (859) 335-5128 www.parkerorings.com
5.11 Friction Friction, either break-out, running, or both, can become troublesome in some applications. At any given time, there are anomalies and diffi culties in the prediction of developed friction. These are accentuated if one of the surfaces involved is deformable as in O-ring piston or shaft seals. An understanding of the principles may prove helpful in the solution of specifi c problems. 5.11.1 Break-Out Friction In addition to the usual causes of running friction: hardness of the rubber, type of surface, surface fi nish, squeeze on the O-ring, amount and type of lubrication, fl uid pressure/ temperature, the amount of break-out friction that a system will generate depends on the length of time the surfaces of the metal and the seal element have been in physical contact at rest. See Figures 5-7 and 5-8. The theory has been proposed and generally accepted that the increase of friction on standing is caused by the rubber O-ring fl owing into the microfi ne grooves or surface irregularities of the mating part. As a general rule for a 70 durometer rubber against an 8 micro-inch surface, the maximum break-out friction that will develop in a system is 3 times the running friction. This ratio can be reduced by the use of a softer rubber. Table 5-3 shows some of the factors which may be used to adjust friction. Coeffi cient of friction has little bearing on lubricated rubber’s break-out and running friction. The other variables listed are much more important in the practical solution to problems. Pounds of Friction Steel 8 Micro-In. RMS Glass Running Friction with 15 Feet per Minute Stroke Speed 2-1/2 Sec. 40 Min. 300 Hrs. Delay Between Cycles Figure 5-7: Change of O-ring Friction with Time at Rest 2-1/2 Sec. 40 Min. with Lubrication Figure 5-8: Flow of O-Ring into Metallic Surfaces 300 Hrs. Parker O-Ring Handbook 5.11.2 Running Friction High running friction may cause diffi culty by wearing soft metal parts. Metals such as copper, brass or aluminum can be rapidly worn away by a moving O-ring. This is especially true if high pressures are involved. If unexplained leakage occurs with these or other soft metals, it is good practice to check the metal dimensions for signs of wear. The following formulas may be used for estimating the running friction of O-rings. Piston Groove Rod Groove FC = fc x Lp FC = fc x Lr FH = fh x Ap Fh = fh x Ar F = FC + FH F = Fc + FH Ap = Projected area of seal for piston groove applications. Ar = Projected area of seal for rod groove applications. F = Total seal friction in pounds. FC = Total friction due to seal compression. FH = Total friction due to hydraulic pressure on the seal. fc = Friction due to O-ring compression obtained from Figure 5-9. fh = Friction due to fl uid pressure obtained from Figure 5-10. Lp = Length of seal rubbing surface in inches for piston groove applications. Lr = Length of seal rubbing surface in inches for rod groove applications. Example: Parker 2-214 rubbing against OD of O-ring at 103.5 Bar (1500 psi), 10% compression, 70 durometer: FC = 0.7 x 3.93 = 2.75 FH = 48 x 0.44 = 20.90 F = FC + FH = 23.65 pounds Data for the coeffi cients (fc and fh) are given in Figures 5-9 and 5-10. Projected areas and lengths of rubbing surface are given in Table 5-4. Friction Factors (In Order of Importance) To Increase To Decrease Friction Factor Friction Increase Unit Load (squeeze) Decrease Increase RMS Surface Finish (metal) Decrease RMS Increase Durometer Decrease Decrease Speed of Motion Increase Increase Cross Section of O-Ring Decrease Increase Pressure Decrease Omit Lubrication Lubrication Use Lubrication Decrease Temperature Increase Decrease Groove Width Increase Increase Diameter of Bore or Rod Decrease Decrease Surface Finish (O-Ring) Increase Stretch O-ring Joule Effect* Compress O-Ring Lower Durometer Coeffi cient of Friction# Increase of O-ring * Refer to rotary seals. Durometer # A minor factor and should be ignored in design work other than for ultra high speeds. Table 5-3: Friction Factors Parker Hannifi n Corporation • O-Ring Division 2360 Palumbo Drive, Lexington, KY 40509 Phone: (859) 269-2351 Fax: (859) 335-5128 www.parkerorings.com 5-7 Dynamic O-Ring Sealing
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aerospace climate control electrome
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50 th 50 th 50 th Anniversary Editi
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Section I - Introduction 1.0 How to
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1.4 Operation All robust seals are
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1.7.1 Static Seals In a truly stati
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1.11 Comparison of Common Seal Type
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Section II - Basic O-Ring Elastomer
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2.1.2 Rubber Rubber-like materials
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2.2.6 Chloroprene Rubber (CR) Chlor
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2.3 Compound Selection and Numberin
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O-ring seal, whether static or dyna
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Percent Compression 40% 30% 20% 10%
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Percent Compression 40% 30% 20% 10%
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Compression Set (% ) 100 Figure 2-1
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2.4.12 Thermal Effects All rubber i
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coeffi cient of expansion for that
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2.6 Aging Deterioration with time o
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A well-known rapid method for mater
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2.13.2 Temperature Temperature rang
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100 80 60 40 20 0 °C -50 °F -58 -
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misapplication might be greatly red
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c. Elongation Change Experience wil
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is possible to receive a part with
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Section III - O-Ring Applications 3
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3.1.2 Temperature Operating tempera
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O-Lube has some unique advantages.
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Parker Boss Kit Sizes Size Dimensio
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Recovery Percent of Original Delect
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3.9.5 Fuels for Automobile Engines
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Parker Seal produces a number of co
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Page 89 and 90: Guide for Design Table 4-2 If Desir
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Page 119 and 120: 5.15 Spiral Failure A unique type o
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Page 129 and 130: Parker O-Ring Handbook Gland Design
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Page 137 and 138: X Gland Detail 0° to 5° Break Cor
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Page 143 and 144: Parker O-Ring Handbook Design Table
Page 145 and 146: 5.31.4 O-Ring Glands for Rotary Sea
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COMPOUND COMPATIBILITY RATING 1 - S
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Section VIII - Specifi cations 8.1
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Military Fluid Specifi cation Descr
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AMS (1) and NAS (2) Rubber Specifi
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Parker O-Ring Handbook Compound Sel
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Section IX Sizes Parker Series 2-XX
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Table 9-1: Parker Series 2-XXX O-Ri
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Parker O-Ring Handbook Parker Serie
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Parker Series 5-XXX O-Ring Sizes (C
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Parker Series 5-XXX O-Ring Sizes (C
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Series 5-XXX Locator Table Size I.D
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Parker O-Ring Handbook Inside Diame
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JIS B2401 Sizes JIS Thickness Inner
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Parker O-Ring Handbook Unusual Size
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Parker O-Ring Handbook Unusual Size
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Section X - Appendix 10.1 O-Ring Fa
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10.1.1.2 Extrusion and Nibbling Ext
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10.1.1.6 Installation Damage Many O
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Parker Seal also has the capability
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10.3 Glossary of Seal and Rubber Te
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Flash: Excess rubber left around ru
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(b) REP (Roentgen equivalent-physic
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10.4 Abbreviations ACM Polyacrylate
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Compound Shrinkage Class Compound N
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Table 10-8: Dimensions From Standar
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Parker O-Ring Handbook Dimensions F
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Appendix 10-32 Parker O-Ring Handbo
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Index — A — Abbreviations. . .
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International O-Ring Standards and
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Offer of Sale 1. Terms and Conditio
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Parker’s Total inPHorm Take the g
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